1989 loses power at higher rpm

on my 1989 325i when replacing my cylinder head I replaced everything in the way h2o pump, distributorcap, rotor.plugs wires. the car ran great for a couple of days but now upon hitting 4000rpm sometimes 5000(especially in higher gears and always under a load) I have a sudden loss of power(feels almost like a loss of spark)and during the the loss of power the check engine light comes on. I dont always have this loss of power but often enough that i cant accelerate like i want to. thanks for any input